This invention relates to methods of detection and use of polarized spin transport in semiconductors and semiconductor devices.
Semiconductor devices rely upon two different types of charge carriers, electrons and holes. The concentrations of these two types of carriers can be controlled by doping intrinsic semiconductors, such as silicon, germanium or gallium arsenide, with electron donors or electron acceptors. This doping creates n-type or p-type semiconductors that are the building blocks of diodes, transistors, photocells and many other high speed, compact integrated circuits used in information processing, logic, control and communications circuits. Semiconductor efficiency decreases below a certain device size, which depends upon (1) diffusion length that determines the depth of the doped layer, (2) the screening length that determines the distance over which a charge perturbation is neutralized, and (3) the small charge carrier concentrations in a semiconductor that lead to shot noise and reduced signal-to-noise ratios. The behavior of a metal is distinguishable from semiconductor behavior: a metal does not depend upon doping layers, has a much smaller screening length, has much larger charge carrier concentrations, and thus has a lower size limit than does a semiconductor.
In order to fabricate a metallic device that has the versatility of a semiconductor device, one needs an approach for distinguishing between two populations of charge carriers in a metallic device. In many ferromagnetic materials, including ferromagnetic metals, ferromagnetic oxides, some ferromagnetic-semiconductors and some insulators, charge transport occurs by two substantially independent channels, the spin-up channel and spin-down channel, each having different electrical conductivities. These two classes of conductivities can be modified independently, using alloying. New devices based on these two sets of charge carriers in magnetic metals have revolutionized the information storage industry in the early 1990s, through use of magnetoresistive read heads that are significantly more sensitive than, and smaller than, the preceding generations of inductive thin film read heads. This trend continues with the recent introduction of spin valve heads based on the giant magnetoresistive effect. Also, spin tunnel junctions, in which tunneling between two metal electrodes is different fro spin-up and spin-down electrons, are being developed for transducers and for magnetic random access memories.
Presently, some workers are focusing on means of fabricating ferromagnetic and semiconductor components for a new class of devices that combine the spins of a ferromagnetic metal, injected into a semiconductor, with many known functions of a semiconductor device.
What is needed is an approach for separately monitoring transport of electrons with polarized spins (up/down) that are injected into and tracked in a semiconductor material.
These needs are met by the invention, which provides a method for estimating the concentration of free electrons with a selected spin polarization in a semiconductor material. In one approach, a static magnetic field with a selected field strength is impressed on a semiconductor in a first selected direction on a semiconductor material. A selected time varying electromagnetic field (e.g., a microwave field) is impressed on the material in a direction transverse to the first selected direction. One or more spin-polarized free electrons from an adjacent ferromagnetic material (metal, oxide, semiconductor or other) is introduced into the semiconductor, and the magnetic moment is allowed to move under the influence of the magnetic field and electromagnetic field. A Hall voltage is measured in a second selected direction across the semiconductor at two or more selected, spaced apart locations in a direction parallel to the second selected direction. The measured Hall voltages are then analyzed to estimate the concentration of free electrons with a selected spin polarization within the semiconductor at one or more of the two or more selected locations. In some embodiments, an oxide film (non-magnetic or magnetic) is positioned between the ferromagnetic material and the semiconductor material.